The invention relates to reducing electromagnetic emissions in electronic devices.
Some Universal Serial Bus (USB) devices use crystal-less oscillators to generate clock signals while providing proper communication and compliance with the USB standard. Instead of using a relatively expensive crystal oscillator, the crystal-less oscillator derives a clock signal internally from a USB data stream.
These crystal-less oscillators maintain a substantially constant clock frequency by locking-in to the USB data stream sent by the USB host. Generally these crystal-less oscillator circuits cannot vary the frequency beyond the range allowed by the USB system, since this would lead to communication failures.
Unfortunately, maintaining a substantially constant operating frequency increases the undesirable effects from electromagnetic emissions both for devices next to the USB device and for circuitry within the USB device. As a result, much of the cost saving from crystal-less oscillator circuits are lost to the additional electromagnetic shielding that is used in the USB device.